U.S. Pat. No. 4,188,778 issued Feb. 19, 1980 to P. Wuthrich discloses a timepiece comprising a case with a circular engagement surface on a back side defining a rear access opening in the case and a back cover member having a peripheral engagement surface of noncircular, multilobed profile adapted to snap-fit to the circular case engagement surface. As a result of its multilobed nature, the back cover member is in snap-fit relation to the circular case engagement surface at only multiple spaced locations corresponding in number to the number of lobes. This type of engagement facilitates removal of the back cover member from the case.
Noncircular caseback configurations are also disclosed in the Miyashita U.S. Pat. No. 3,696,608 issued Oct. 10, 1972 and the Miyasaka U.S. Pat. No. 3,940,922 issued Mar. 2, 1976.
Recent trends in the styling of wristwatches, especially digital wristwatches, have shown a preference for shaped or non-circular cases of generally smaller dimensions. Unfortunately, this trend has limited the amount of space available for a rear access opening and engagement surface on the back side of the watchcase. The size of circular access openings and engagement surfaces has been particularly restricted. Prior art workers have in some situations opted to use shaped access openings and engagement surfaces but these are disadvantageous from the standpoint of increasing manufacturing and machining costs of the case and back cover member. What is needed is an engagement system for the case and back cover member which can be accommodated on smaller, shaped cases and which can be manufactured with conventional equipment and machining procedures.